Detection of Mooney faces is robust to image asymmetries produced by illumination.

Face detection relies on the visual features that are shared across different faces. An important component of the basic spatial configuration of a face is symmetry around the vertical midline. Although human faces are structurally symmetrical, they can be asymmetrical in an image due to the direction of lighting or the position of the face. In the experiments presented here, we examined how face detection from simple contrast patterns that occur across the face is affected by the image asymmetries associated with variations in the horizontal lighting direction. We presented observers with two-tone images of faces (Mooney faces) that isolated the unique pattern of contrast in the shading and shadows on a face, illuminated from a wide range of horizontal directions. In two experiments, we found that face detection is surprisingly robust to these lighting changes, with sensitivity in discriminating between face and non-face patterns reduced only at the most extreme lighting directions. This tolerance to changes in the horizontal lighting direction depended partly on the orientation of the face, vertical lighting direction, and contrast polarity. Our results provide insight into how contrast cues produced by shading and shadows occurring across the facial surface are utilized by the visual system to detect human faces.

Estimating lighting direction in scenes with multiple objects.

To recover the reflectance and shape of an object in a scene, the human visual system must account for the properties of the light illuminating the object. Here, we examine the extent to which multiple objects within a scene are utilised to estimate the direction of lighting in a scene. In Experiment 1, we presented participants with rendered scenes that contained 1, 9, or 25 unfamiliar blob-like objects and measured their capacity to discriminate whether a directional light source was left or right of the participants' vantage point. Trends reported for ensemble perception suggest that the number of utilised objects-and, consequently, discrimination sensitivity-would increase with set size. However, we find little indication that increasing the number of objects in a scene increased discrimination sensitivity. In Experiment 2, an equivalent noise analysis was used to measure participants' internal noise and the number of objects used to judge the average light source direction in a scene, finding that participants relied on 1 or 2 objects to make their judgement regardless of whether 9 or 25 objects were present. In Experiment 3, participants completed a shape identification task that required an implicit judgement of light source direction, rather than an explicit judgement as in Experiments 1 and 2. We find that sensitivity for identifying surface shape was comparable for scenes containing 1, 9, and 25 objects. Our results suggest that the visual system relied on a small number of objects to estimate the direction of lighting in our rendered scenes.

Discrimination of facial identity based on simple contrast patterns generated by shading and shadows.

The pattern of shadows and shading across a face is determined partly by face shape and may therefore provide a cue for facial recognition. In this study, we measured the ability of human observers to discriminate facial identity based simply on the coarse pattern of contrast produced by the interaction between facial geometry and lighting direction. We used highly realistic 3D models of human heads to create images of faces illuminated from different horizontal and vertical directions, which were then converted to two-tone images ('Mooney faces') to isolate the coarse pattern of contrast. Participants were presented with pairs of two-tone faces and judged whether it was the same person in both images. Participants could discriminate facial identity based on the minimal cues within the two-tone images, though sensitivity depended on the horizontal and vertical lighting direction. Performance on the Mooney recognition task correlated with general facial recognition ability, though the role of face-specific processing in this relationship was not significant. Our results demonstrate that shading information in the form of simple contrast cues is sufficient for discriminating facial identity, and support the idea that visual processing is somewhat optimised for overhead lighting - here, in the relatively high-level context of face identity recognition.

Surface curvature from kinetic depth can affect lightness.

The light reaching the eye confounds the proportion of light reflected from surfaces in the environment with their illumination. To achieve constancy in perceived surface reflectance (lightness) across variations in illumination, the visual system must infer the relative contribution of reflectance to the incoming luminance signals. Previous studies have shown that contour and stereo cues to surface shape can affect the lightness of sawtooth luminance profiles. Here, we investigated whether cues to surface shape provided solely by motion (via the kinetic depth effect) can similarly influence lightness. Human observers judged the relative brightness of patches contained within abutting surfaces with identical luminance ramps. We found that the reported brightness differences were significantly lower when the kinetic depth effect supported the impression of curved surfaces, compared to similar conditions without the kinetic depth effect. This demonstrates the capacity of the visual system to use shape from motion to "explain away" alternative interpretations of luminance gradients, and supports the cue-invariance of the interaction between shape and lightness. (PsycINFO Database Record (c) 2018 APA, all rights reserved).